1. Field of the Invention
The present invention relates to a transmission unit and a two-way signal conversion method. More particularly, the present invention relates to a transmission unit which transports digital signals, and to a method to convert signal formats, transmission rates, hierarchical levels, and other attributes in both directions.
2. Description of the Related Art
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) specifications have been standardized and implemented as today's core data multiplexing technologies which provide an efficient way of integrating a wide variety of high-speed and low-speed services. Digital signals that this type of synchronous networks carry are organized in accordance with prescribed hierarchical multiplexing levels, where lower-order group signals (i.e., slower signals) are combined into a higher-order signal (i.e., faster signal). Each network element accepts such higher-order multiplexed signals form the upstream element and retransmits them the downstream element, while extracting and inserting some lower-order signals. Some nodes with cross-connect capabilities provide consolidation and segregation of transport signals.
As an approach for improved maintainability of the above transport systems, a signal loopback technique is disclosed in the Japanese Unexamined Patent Publication No. 10-243008 (1998). According to the disclosed technique, the signal in any specified time slot of a low-order group channel is looped back to another low-order group channel. This conventional technique, however, is limited to low-order group signals of the same hierarchical series of multiplexing levels, and it is therefore not possible to loop back a signal from one channel to a different type of channel, such as from a high-order SONET channel to a high-order SDH channel. That is, conventional systems do not support looping back between networks having different signal specifications. For operations and maintenance of various telecommunication services on a global scale, it is necessary to develop an improved transport system which supports efficiently converting, not only between SONET and SDH, but other signals having different specifications to one another.